The present invention relates to an electromagnetic brake and an automotive driving force distributing apparatus utilizing the same electromagnetic brake.
A differential is disposed along a power train of a vehicle so as to allow an outer wheel to rotate faster than an inner wheel during turning of the vehicle while keeping a torque distribution in which a torque is equally divided between the left and right wheels to thereby obtain a smooth turning in an ensured manner.
While a main function of the differential is to allow the vehicle to have a smooth turn on a curved road, what would happen, for example, in case only one of the wheels is trapped in a muddy spot to thereby start spinning while the vehicle is running on a rough road?
A small resistance is applied to the wheel trapped in the muddy spot and almost all the rotational force is transferred to the spinning wheel, whereby no traction is transferred to the other wheel. Thus, as a whole, the traction becomes too short for the wheel to get out of the muddy spot, which is a drawback of the differential in general.
It is a differential with a differential motion limiting mechanism that is intended to prevent the drawback, and the differential with a differential motion limiting mechanism has a function to compensate for the basic drawback which is inherent in the aforesaid differential. The differential of this type is referred to as a limited slip differential (LSD).
In general, the conventional differential is constituted by a planetary gear-type differential, and a planetary gear-type differential gear assembly is disclosed in JP-A-6-33997 which has a differential motion limiting mechanism including an electromagnetic clutch and a multi-plate clutch.
In this differential gear assembly, an attracting force between a solenoid and an armature of the electromagnetic clutch is applied to the multi-plate clutch so as to press against the multi-plate clutch to thereby selectively control the engagement force of the multi-plate clutch.
A connecting member having a plurality of legs is disposed between pressure plates of the multi-plate clutch and the armature. One ends of these legs are fixed to the pressure plates of the multi-plate clutch, and the other ends thereof are brought into abutment with inner circumferential portions of the armature when the solenoid is actuated.
In the aforesaid differential gear assembly, the plurality of legs are fixed to the pressure plates and extend in substantially a perpendicular direction to the pressure plates. Therefore, in the event that some of these legs are attached to the pressure plates in an inclined fashion, there may be caused a problem that a pressing force applied by the armature which is attracted by the solenoid is not uniformly transferred to the pressure plates of the multi-plate clutch.
Furthermore, in the differential gear assembly disclosed in the aforesaid unexamined patent publication, since the engagement force of the multi-plate clutch is controlled by the electromagnetic clutch, the plurality of legs or bars functioning as the press members are disposed so as to correspond to the inner circumferential portions of the armature.
In a multi-plate brake construction, however, brake plates and brake discs are generally disposed on an outer circumferential side when considering the construction of the brake apparatus. Therefore, it is difficult to apply the construction disclosed in the unexamined patent publication in which the armature and the multi-plate clutch are operatively connected to each other on the inner circumferential side to the multi-plate brake construction.
Hereinafter, described is a brief explanation of the related art, but not of a prior art for the present invention. The applicant of the invention previously proposed (in Japanese Patent Application No. 2001-267785 or its corresponding U.S. patent application ser. No. 10/198,176 (published as 20030075395) an electromagnetic brake which could solve the aforesaid problem. The electromagnetic brake according to the previous patent application includes a multi-plate brake mechanism, a ring-like core member having an annular exciting coil and a ring-like armature member disposed so as to correspond to the annular exciting coil of the core member.
The electromagnetic brake includes further a cylindrical pressure member which is fixed to an outer circumferential portion of the armature member at one end and is engaged with the multi-plate brake mechanism at the other end thereof and which is fitted on the core member in such a manner as to be guided by the core member so as to move in a direction in which the multi-plate brake mechanism is pressed.
The electromagnetic brake according to the previous patent application provides a construction in which an air gap is defined between the armature member and the core member, and the armature member is attracted by energizing the exciting coil so that the multi-plate brake mechanism is brought into engagement, and the thrust of the exciting coil is largely affected by the extent of the air gap.
The cylindrical press member of this electromagnetic brake has a function to transfer the thrust of the exciting coil to the multi-plate brake mechanism, as well as a function to implement the positioning of the armature member in a radial direction so that the armature member can be displaced uniformly in a circumferential direction.
In the electromagnetic brake according to the previous patent application, since the radial positioning accuracy of the armature member is designed to take priority, the cylindrical press member needs to be formed of a material of similar system/type to that of the core of the exciting coil in consideration of linear expansion due to change in temperature. In the electromagnetic brake according to the previous patent application, the cylindrical press member may be formed of a stainless steel.
Since the housing may be formed of an aluminum alloy in order to reduce the weight of the electromagnetic brake, the linear expansion of the housing does not match that of the cylindrical press member. When considering the electromagnetic brake as a whole, the air gap between the armature member and the core member changes as the temperature changes. There is a possibility that this may lead to a problem that the engagement force (braking force) of the multi-plate brake mechanism is caused to fluctuate.
Consequently, in the event that this electromagnetic brake is applied to an automotive driving force distributing apparatus, there is caused a problem that the implementation of appropriate driving force distribution becomes difficult as the temperature changes.
Thus, an object of the invention is to provide an electromagnetic brake which can restrain a fluctuation in the extent of the air gap between the armature member and the core member from occurring as the temperature changes.
According to a first aspect of the invention, there is provided an electromagnetic brake interposed between a stationary housing and a rotational member which is accommodated at least partially within the stationary housing.
The electromagnetic brake includes a multi-plate brake mechanism having a plurality of brake plates attached to the stationary housing and a plurality of brake discs attached to the rotational member in such a manner as to be interleaved with the plurality of brake plates, a ring-like core member having an annular groove and a first outside diameter and fixed within the housing, an annular exciting coil accommodated in the annular groove in the core member and a ring-like armature member having a second outside diameter which is larger than the first outside diameter and disposed in such a manner as to confront the annular groove in the core member.
The electromagnetic brake includes further a cylindrical press member having a first end and a second end and fitted on the core member in such a manner as to freely move in a direction in which the multi-plate brake mechanism is pressed, the first end being fixed to an outer circumferential portion of the armature member, the second end being brought into engagement with the multi-plate brake mechanism and a cylindrical guide member fixed to an inner circumferential portion of the armature member at one end thereof and fitted in the core member so that the armature member is guided by the core member so as to move in the direction in which the multi-plate brake mechanism is pressed.
According to the electromagnetic brake of the first aspect of the invention, since the cylindrical guide member is provided which is fixed to the inner circumferential portion of the armature member and is fitted in the core member, the radial positioning of the armature member is attained by the cylindrical guide member.
As a result, since the cylindrical press member does not have to include the function to implement the radial positioning of the armature, it is possible to select a material and to design the cylindrical press member while considering thermal expansion.
According to a second aspect of the invention, there is provided an electromagnetic brake, wherein the housing and the cylindrical press member are formed from an identical material. Since the housing and the cylindrical press member are formed of the identical material, it is possible to suppress a change in the extent of air gap between the armature member and the core member due to a change in temperature.